At the University of Washington Primate Center, various macaque species show an immune deficiency syndrome (SAIDS) characterized by lymphocytopenia, opportunistic infections, and a fibromatosis tumor (RF). A type, D retrovirus was isolated by cocultivation of explants of RF tissue from a rhesus monkey (Macaca mulatta). This isolate, designated SAIDS-D/Washington, grows to high titers in a variety of primate and mammalian cells and morphologically transforms various rodent cell lines. The SAIDS-D virus can be distinguished from all other retroviruses by antigenicity and molecular hybridization, but is partially related to Mason-Pfizer and the langur monkey type D viruses. Nuclei acid hybridization studies reveal the presence of multiple copies of sequences partially related to the SAIDS-D virus in all Old World monkey cellular DNAs. The highest extent of homology is detected in langur monkey DNA. Since macaques and langurs cohabit Southeast Asia, that region may be reservoir for SAIDS. Eight polypeptides corresponding to gag and env viral regions have been purified by HPLC. Radioimmunoassays developed with these proteins reveal that SAIDS-D/Washington virus can be distinguished from all other primate isolates, including type D viruses isolated at the California and New England Primate Centers. Additional SAIDS-D isolates have now been obtained from M. nemestrina, M. fuscata, and M. fascicularis with symptoms of SAIDS or with biopsy-confirmed RF. Eight macaques have been inoculated with SAIDS-D virus; to date one animal has died with symptoms of SAIDS, and one other macaque, known to be viremic, has developed the palpable abdominal nodules characteristic of RF. Vaccinations are being performed with disrupted virus and purified proteins in attempts to prevent SAIDS. The identification of a retrovirus associated with simian AIDS, the ability to cause the disease with this virus, and the prevention of disease by vaccination will have strong and immediate implications for the human disease and its control.